Infrared radiation emitting ceramic material

ABSTRACT

The present invention relates to ceramic materials that are capable of generating continuous infrared radiation and have enhanced thermal, chemical and physical stability. The object of the invention is to raise the efficiency of emitting IR radiation. The goal is achieved by supplementing the material on the basis of Cr 2 O 3  and Fe 2 O 3 , with CaCO 3 .

The present invention relates to novel ceramic materials that are capable of generating continuous infrared (IR) radiation. More particularly, it relates to refractory ceramic materials that have enhanced thermal, chemical, and physical stability as well as enhanced resistance to thermocycling and that can be used for drying things, for quickly and efficiently sterilizing various objects, for treating cardiovascular disorders, for conveying gases and liquids, for air purification, in cooling systems, in systems where conductivity varies in pulses, in high-temperature heaters, and in lining materials.

Ceramic materials are known of the general formula RCrO₃, where R is a rare-earth oxide such as yttrium oxide, described in U.S. Pat. No. 3,475,352. These materials, although successfully used where conductance is essential, e.g. in electrodes, have low chemical stability when exposed to high (over 1600° C.) temperatures, low resistance to thermocycling at temperatures above 1500° C., and cannot be heated at high rates. Furthermore, they have relatively low radiating capacity. These shortcomings have limited the use of these materials in applications where the stability of properties is important.

A device is known for sterilizing a hose coupling for an artificial kidney (see U.S. Pat. No. 4,774,415, Class 250-455.1). The ceramic material of the sterilizer has an insufficient radiating power in the infrared range, which results in insufficient sterilization of some parts of the coupling which do not get the necessary exposure. Increasing the time of sterilization does not help, since it may result in overheating the parts of the coupling most susceptible to TR radiation. In this case articles made of metal and glass may melt, or change colour due to the formation of an oxide film.

A ceramic Composition having more stable thermal, physical and chemical properties is one based on chromium oxide Cr₂O₃ (up to 44.7 wt %) and iron oxide Fe₂O₃ (up to 35 wt %) and also containing a large amount of silicon oxide SiO₂ (10˜20 wt %) and Al₂O₃, CuO, CaO and MgO as additives (see U.S. Pat. No. 5,350,927, published 27.09.1994, national classification 250-504R, Radiation emitting ceramic materials and devices containing the same), chosen as the prototype. The said ceramic material receives either pulse IR radiation from another ceramic material generating only pulse IR radiation, made in the form of a screen, or radiation from a lamp, a heating coil, and so on, and generates continuous IR radiation used in sterilizers. A shortcoming of the ceramic is the relatively low efficiency of conversion of primary radiation, which severely limits its application in the many fields listed in the preamble.

The object of the present invention is to create a ceramic material that will be capable of uniform IR radiation and have an increased radiating capacity. This goal is achieved by supplementing a ceramic material containing Cr₂O₃, Fe₂O₃, SiO₂, MgO, Al₂O₃, CaO, and CuO, with CaCO₃, in the following ratio of ingredients, wt %:

Cr₂O₃ 28.0 ˜ 32.0 CaCO₃  7.0 ˜ 10.0 Fe₂O₃ 33.0 ˜ 35.0 SiO₂ 16.0 ˜ 17.5 MgO 4.0 ˜ 6.0 CaO 2.5 ˜ 3.0 Al₂O₃ 0.5 ˜ 2.0 CuO 0.5 ˜ 1.0

It has turned out that it is precisely the addition of CaCO₃ in the amount given, with a simultaneous decrease, relative to the prototype, in Cr₂O₃, CaO and CuO to the amounts indicated and the amounts of the remaining ingredients held constant, that allows one to obtain a ceramic material capable of generating continuous IR radiation with a radiating power exceeding that of known ceramic materials on the basis of chromium oxide by 23 to 49%.

The ceramic material being claimed is prepared following a conventional route. The ingredients arc milled to a fine powder, mixed until homogeneity is achieved, and placed in the crucible of a solar furnace, where they are melted. The melting is usually done in conditions where the loss of oxygen from the resulting powder is kept to a minimum. As a rule, these ceramic compositions on the basis of chromium oxide are melted at about. 2 600° C. It is desirable that the melting be carried out in an oxidizing atmosphere, preferably in air.

The melt drips into a cooling tank containing water. The resultant material is ground to a grain size of 160 μm and then the powder is pressed into desired articles.

The invention is illustrated in more detail with the following examples of preparing the ceramic material.

EXAMPLE 1

To 1 kg of slag dump from the copper foundry within the Almalik mining and smelting works, containing 274.59 g SiO₂, 600.67 g Fe₂O₃, 34.32 g Al₂O₃, 8.58 g CuO, 51.5 g CaO and 11 g MgO, were added 497.7 g Cr₂O₃, 154.5 g CaCO₃ and 83.39 g MgO, or, given in wt %:

Cr₂O₃ 29.0 CaCO₃ 9.0 Fe₂O₃ 35.0 SiO₂ 16.0 MgO 5.5 CaO 3.0 Al₂O₃ 2.0 CuO 0.5

Then the substances were ground in a planetary mill having drums with a plastic lining and tenon balls to a fine powder. The powder was put in the crucible of a solar furnace and melted at 2 600° C. in air. The melt was then dripped into a cooling tank containing water. The ceramic material thus obtained was again milled to a grain size of 160 μm and pressed into desired articles.

In Examples 2 to 4, specimens of the ceramic material were obtained in a similar manner to that in Example 1, only the ratios of ingredients were different. For ease of explanation the compositions of all the examples are listed in Table 1. It should be added that only within the limits of the ratios indicated was it possible to obtain a material with a higher power of emitting IR radiation when heated than that of the prototype.

TABLE 1 Ingredient, wt % Example 1 Example 2 Example 3 Example 4 Cr₂O₃ 29.0 28.0 32.0 27.0 CaCO₃ 9.0 10.0 7.0 10.0 Fe₂O₃ 35.0 33.0 35.0 34.0 SiO₂ 16.0 17.5 17.0 17.5 MgO 5.5 6.0 4.0 6.0 Al₂O₃ 2.0 1.5 2.0 2.0 CaO 3.0 3.0 2.5 3.0 CuO 0.5 1.0 0.5 0.5

Then the powers of IR radiation emitted by the ceramic material being claimed and the prototype material were measured by the comparison method. For this purpose two planar radiators were used that had the same parameters: 120×24×1.5 mm (an aluminium one) and 120×24×0.8 mm (a copper one). One plate was coated with the prototype material, the other, with the material according to the present invention. Both plates were attached on one side to a small-sized heating element. Each ceramic element was 25×5×1 mm in size and had a resistance of 440 Ohm.

The measurement technique is as follows:

A temperature sensor, together with the heating element, is attached to the centre of the cooling plate using a spring clamp. The heating element is fed with various powers which arc stabilized by the applied voltage. The energized heating element is kept until an equilibrium thermal state is attained at a given environment temperature (natural convection), an indication of which is the constancy in time of the temperature measured by the thermocouple.

Thus, by subtracting the thermometer readings taken on the plate coated with the prototype ceramic from those taken on the plate coated with the ceramic according to the present invention, the difference in temperature is obtained that characterizes the effect of the ceramic only, since the radiating power is in both cases the same.

Two materials were compared that had different thermal conductivities:

an aluminium alloy with a conductivity of 150 W/(m K), and

a copper alloy with a conductivity of 350 W/(m K).

The voltage applied to the heating element is alternating, with a frequency of 50Hz.

In all the instances as a prototype was used composition C according to U.S. Pat. No. 5,350,927, with the maximum Cr₂O₃ content:

Results of measuring radiating power on the aluminium alloy for the ceramic of Example 1:

TABLE 2 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic platecoated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 78 69  9 50 5.682 87 77 10 55 5.875 97 85 12 60 8.182 104 91 13 65 9.602 113 98 15 70 11.136 121 106 15 75 12.784 129 114 15 80 14.545 137 121 16 85 16.421 146 128 18 90 18.409 152 133 18

Results of measuring radiating power on the aluminium alloy for the ceramic of Example 2:

TABLE 3 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 78 70  8 50 5.682 87 79  8 55 6.875 97 87 10 60 8.182 104 94 10 65 9.602 113 101 12 70 11.136 121 108 13 75 12.784 129 116 13 80 14.545 137 124 13 85 16.421 146 131 15 90 18.409 152 137 15

Results of measuring radiating power on the aluminium alloy for the ceramic of Example 3:

TABLE 4 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 78 75 3 50 5.682 87 83 4 55 6.875 97 93 4 60 8.182 104 99 5 65 9.602 113 106 7 70 11.136 121 112 9 75 12.784 129 118 11 80 14.545 137 125 12 85 16.421 146 134 12 90 18.409 152 140 12

Results of measuring radiating power on the aluminium alloy for the ceramic of Example 4:

TABLE 5 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 78 71 7 50 5.682 87 79 8 55 6.875 97 87 10 60 8.182 104 94 10 65 9.602 113 102 11 70 11.136 121 109 12 75 12.784 129 117 12 80 14.545 137 124 13 85 16.421 146 131 15 90 18.409 152 137 15

It is quite obvious from Tables 2 to 5 that, although fed with the same the plate coated with the ceramic according to the present invention gets heated to a lesser extent owing to the higher efficiency of emitting (dissipating) the radiation received.

The principal unit of power in the radiometric system is Watt. It is the measure of energy emitted per unit time. The difference in the power fed to the heating element, at some fixed temperatures, clearly shows the efficiency of emission of the input power by the ceramic according to the present invention.

TABLE 6 Power fed to Power fed to the the plate plate covered with covered with the ceramic according Temperature, the prototype to the present P₂-P₁, P₂-P₁, ° C. ceramic, P₁, W invention, P₂, W W %  91 6.202 8.182 1.98 30.0 114 9.602 12.784 3.182 24.9 121 11.136 14.545 3.409 23.4

Results of measuring radiating power on the copper alloy for the ceramic of Example 1:

TABLE 7 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 81 65 16 50 5.682 90 73 17 55 6.875 99 82 17 60 8.182 107 89 18 65 9.602 116 96 20 70 11.136 125 103 22 75 12.784 134 112 22 80 14.545 143 119 24 85 16.421 150 126 24 90 18.409 159 132 27

Results of measuring radiating power on the copper alloy for the ceramic of Example 2:

TABLE 8 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 81 68 13 50 5.682 90 76 14 55 6.875 99 85 14 60 8.182 107 92 15 65 9.602 116 100 16 70 11.136 125 106 19 75 12.784 134 114 20 80 14.545 143 122 21 85 16.421 150 128 22 90 18.409 159 135 24

Results of measuring radiating power on the copper alloy for the ceramic of Example 3:

TABLE 9 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 81 72 9 50 5.682 90 80 10 55 6.875 99 89 10 60 8.182 107 96 11 65 9.602 116 104 12 70 11.136 125 113 12 75 12.784 134 119 15 80 14.545 143 128 15 85 16.421 150 133 17 90 18.409 159 141 18

Results of measuring radiating power on the copper alloy for the ceramic of Example 4:

TABLE 10 Temperature T₂ Temperature T₁ of the plate coated of the with the ceramic plate coated according to the Voltage, Power, with prototype present V W ceramic, ° C. invention, ° C. T₁-T₂, ° C. 45 4.602 81 70 11 50 5.682 90 78 12 55 6.875 99 86 13 60 8.182 107 94 13 65 9.602 116 102 14 70 11.136 125 109 16 75 12.784 134 117 17 80 14.545 143 126 17 85 16.421 150 131 19 90 18.409 159 137 22

It can be seen from Tables 7 to 10 that, with the copper alloy, the coating of the ceramic being claimed is even more efficient in emitting the power received, which, in turn, results in a greater difference in the temperatures to which copper plates coated with the ceramics are heated when emitting the same power.

Some examples of comparing the emitted powers expressed in percentages reveal an even more striking difference in the efficiency of power emission by the ceramic according to the present invention and the prototype ceramic.

TABLE 11 Power fed to the plate Power fed to covered with the plate the ceramic Heating covered with according to temperature, the prototype the present ° C. ceramic, P₁, W invention, P₂, W P₂-P₁, W P₂-P₁, %  81 4.602 6.875 2.273 49.0  90 5.682 8.182 2.500 44.0 125 11.136 16.421 5.285 47.5

The data given clearly show an increase in the efficiency of emitting IR radiation by the novel ceramic material of up to 50% as compared with the prototype material. It can also be seen that Example 1 gives the optimum composition that ensures the highest efficiency of emitting IR radiation. The other compositions also contribute to increasing the power of IR radiation, but to a lesser extent. Going outside the range of the claimed ingredient percentages of the novel ceramic material in either direction results in there being no such effect or its being so small as to be of no practical importance.

Notwithstanding that only four examples are given illustrating the percentage composition of the novel ceramic material, so as not to clutter the description with dozens of tables, the scope of the invention covers all the combinations of the ingredients falling within the given percentage ratios. 

What is claimed is:
 1. An infrared ceramic material comprising Cr₂O₃, Fe₂O₃, SiO₂, MgO, Al₂O₃, CaO, and CuO, whose distinguishing property is that it also contains CaCO₃, in the following ratios of ingredients, in wt %: Cr₂O₃ 28.0˜32.0 CaCO₃  7.0˜10.0 Fe₂O₃ 33.0˜35.0 SiO₂ 16.0˜17.5 MgO 4.0-6.0 CaO 2.5˜3.0 Al₂O₃ 1.5˜2.0 CuO  0.5˜1.0. 